The present invention relates to acoustic logging systems and more particularly to acoustic logging systems that are referred to as borehole televiewers. A borehole televiewer refers to an acoustic logging system in which a transducer transmits a beam of high frequency acoustic energy into the borehole in a radial plane and receives the reflected response of the acoustic pulse. The transducer is mounted in a housing so that it may be rotated to scan the complete surface of the borehole. Such systems are also used to scan casing which is used for lining a borehole to determine the condition of the casing.
These systems have been extensively used for scanning the surface of a borehole to determine the lithology of the formations penetrated by the borehole as well as locating bedding planes and similar characteristics of the formations. The system has also been used to determine the quality of the bond between cement and the well casing. This type of system has been described in the prior art and particularly in U.S. Pat. No. 3,369,626 entitled "Method and Apparatus for Producing A Visual Record of Physical Conditions of Materials Transversed by a Borehole".
While the systems have been described in the prior art and have been extensively used, they do have several shortcomings. Since only a single transducer is used to transmit the acoustic pulse into the formation and receive the reflected response of the acoustic pulse, the system does not respond to anomalies which do not lie in a plane that is substantially perpendicular to the direction of the pulse. Fractures in almost all cases lie at some angle to the borehole. Likewise, there may be anomalies or channels in the cement between the formation and the well casing that will allow unwanted flow of fluids behind the well casing.
The aforementioned shortcomings produce ambiguities when the system is used to evaluate fractured formations penetrated by a borehole. While the prior art system locates fractures that penetrate the borehole, it does not evaluate the fracture. In particular, it does not indicate whether the fracture is open or closed. Obviously, if the fracture is closed it will be impossible to produce the formation even though hydrocarbons may be present in the fractures. Thus, accurate evaluation of fractures is important.
U.S. Pat. No. 4,601,024 described a borehole televiewer using multiple vertically spaced transmitting/receiving transducers to improve the performance of the system. The patent describes the tilting of the acoustic beam of the transducers to obtain deeper penetration of the acoustic beam into the formation. The patent does not address the problem of obtaining a response from a feature that is not coplanar with the borehole.